Field Thunder with multibodies for most of the game. Field Thunder is powerful and, most importantly, looks cool.
Ring Blaster really comes in handy in certain situations, like against the stage 7 boss and in stage 9 when all those small enemies come at you from the sides.
Sometimes--like in stage 6 with all its small, fast enemies--I use the wave weapon because it has faster fire and fewer gaps than Field Thunder.
I never really use the standard gun, probably just because I find it kinda boring.